The present invention relates generally to the production of electric power using geothermal fluids and, more particularly, to processes used in a geothermal power plant for clarifying geothermal brine to make it suitable for reinjection into the earth and for producing silica seed material, the seed material being useful for enhancing the precipitation of dissolved silica and agglomeration of suspended solids in brine at other stages in the geothermal power plant.
Geothermal brine, having temperatures of over about 500.degree. F., may be withdrawn from large subterranean aquifers which have been found in many areas of the world. The most common occurrence of aquifers having comparatively easy access thereto is where the earth's near-surface thermogradient is significantly high. Manifestation of readily accessible subterranean aquifers is typically the occurrence of volcanic, fumarole or geyser activity.
Brine and steam from naturally occurring geyser activity have been used for many years, both industrially as a heat source, and commercially as in therapeutic baths, and the like. Extraction wells, drilled into the earth's surface to intercept the subterranean aquifers, can bring a steady dependable supply of hot pressurized brine to the earth's surface whereupon steam is flash extracted from the pressurized brine and thereafter utilized to generate electrical power.
While in principal, the extraction of geothermal brine and the eventual generation of electrical power therefrom may be simple, its implementation is not. Unfortunately, geothermal brine is not only usually saline, from which its name is derived, but contains many dissolved minerals and gases. The capacity of the geothermal brine to cause a large amount of contaminants is due to the extreme temperature (over about 500.degree. F.) and pressure (over about 450 p.s.i.g.) at which it is contained within the subterranean aquifiers. Examples of the dissolved gases and minerals are hydrogen sulfide, carbon dioxide, ammonia, silica, as well as lead, iron, arsenic and cadmium compounds, to name a few.
It should be readily apparent that the dissolved gases and minerals within the geothermal brine are very corrosive to processing equipment. In addition, during the production of steam from the brine, the pressure is reduced, thus causing many of the dissolved minerals and compounds to precipitate out of the brine and onto any available surface within the processing equipment.
The presence of silica is particularly troublesome in this regard. As hereinbefore noted, high pressurized geothermal brine is typically saturated with silica and when the pressure of the liquid is reduced in order to flash extract steam for power production, the liquid becomes supersaturated with silica. Silica precipitation then occurs from the brine, forming a hard scale within the flash crystallization equipment in addition to downstream piping and equipment. The buildup of scale is significant, with scale formation many times occurring at the rate of several inches per month. Scaling of the piping equipment continues to build until the geothermal brine flow through the system becomes restricted and the entire facility must be shut down for reconditioning. As a result of the hardness of the silica scale and its ability to adhere to the inside surfaces, scraping or cleaning such scale is time-consuming and costly, both in terms of actual cost of removal and in terms of facility downtime during which no electrical output is produced.
To minimize the silica scaling problems in geothermal liquid power producing facilities, two general methods have typically been used. The first method is to prevent the silica from precipitation. The second method is to induce precipitation from the geothermal liquid in a controlled manner and at preferred points within the facility and thereafter removing the precipitated silica and other solids before reinjecting the brine into the earth.
The first of these two methods is very difficult to achieve, while extracting as much usable energy from the hot brines as possible. In other words, while precipitation may be prevented by limiting the amount of pressure reduction during flash extraction of steam, it is apparent that all of the steam available from the brine will not be collected unless the flash temperatures are eventually lowered to atmospheric. Hence, the effort to maintain silica in solution results in inefficient extraction of steam from brine.
The second method of silica control is effected through the seeding of the geothermal aqueous brine with a seed material at appropriate points within the facility. The seed material provides an alternative surface onto which the silica can precipitate. Since the original precipitation, or scale, develops on the seed material, adheres thereto and is thereafter surrounded by brine, its subsequent adhesion to the interior surfaces of the equipment is reduced, if not eliminated, thereby allowing the seeded precipitate to be pumped with the brine for later separation thereof.
Seed material for this process usually is obtained from downstage separation of the precipitate and the process typically includes pumping some of the silica precipitate removed from one downstage portion of the system into the flow of geothermal brine at an upstream point. Usually, the injection of the seed material is within a flash crystallizing stage, which may consist of one or more flash crystallization vessels, each one having a progressively lower operating pressure.
As hereinbefore discussed, providing seed materials in the flash crystallization stage of a geothermal power plant facility can significantly reduce the amount of silica scale on the interior surfaces of the vessels. However, the silica precipitate deposited on the seed material and suspended within the brine includes many other mineral and elemental constituents, including lead and arsenic, which cannot be acceptably discharged into the surrounding environment. Ponding and evaporation of the discharged geothermal effluent is also generally impractical, not only because of the large volumes involved, but because of the toxic materials, generated by exposure to oxygen, therein are also considered unacceptable as land-fill or for most disposal facilities. Consequently, the most feasible manner of disposing of geothermal effluent is by pumping it back into the ground through injection wells.
Injection can safely dispose of dissolved materials and is useful in preventing ground subsidence, which might otherwise be caused by depletion of the subterranean aquifiers from which the geothermal brine is initially removed. However, suspended solids, such as silica precipitate, cannot be easily injected back into the earth.
It must be appreciated that the amount of brine extracted and thereafter injected into the earth can be very high. For example, for a 10 megawatt geothermal brine power plant, the brine flow rate can be about 1.2 million pounds per hour. At these flow rates, even a small amount of suspended solids in the injected brine can cause the injection wells to become plugged and thereafter inoperable or inefficient in disposing of geothermal power plant effluent. Reconditioning of a plugged injection well may cost a million dollars, or more, hence, it is imperative to reduce the amount of suspended solids in the geothermal power plant effluent to as low a level as possible.
Typically, suspended solids in brine removed from a flash crystallization stage in a geothermal power plant facility are removed by a clarifier. The clarifier provides an agglomeration zone for coalescing particles with one another and the removal of enlarged solid particles, for later use as seed material. Clarified brine having fine solid particulates therein is thereafter filtered prior to injection into the earth.
Heretofore, the clarifier in a geothermal power plant installation typically produced an underflow having a suspended solids content of about 15 percent by weight. Thereafter, the underflow was processed in a thickener, or settling tank, in order to produce a brine having 25 to 30 percent, by weight solids, or more, which is suitable for seed material in the crystallization stage of the geothermal power plant.
Clarified brine, or overflow, is withdrawn from the clarifier and flowed through a filtering stage, designed for removing most of the fine suspended particulates from the brine prior to reinjection into the earth. Typically, the filtering stage includes a number of filters, interconnected in a series and/or parallel relationship, which are designed to remove suspended particles in the brine larger than about 5 microns in size. Heretofore, the concentration of such particles in the flocculated clarified brine has been between about 70 and 80 parts per million, and the filtration system designed to reduce the amount of such suspended solids to about 10-20 parts per million. Using the state-of-the-art injection well equipment, concentrations of less than 10 parts per million solids enable the injection of such filtered brine into the earth without excessive scale formation and/or injection well plugging.
It must be appreciated that such filtration systems require continual maintenance as they continually become loaded with the solid particulates they remove from the clarified brine and must either be replaced or backwashed to maintain their operational efficiency. The cost of this maintenance is not insignificant. in fact, for a ten megawatt geothermal power plant facility in which the filtration system is removing 70 parts per million suspended solids from a brine flow rate of about 1.2 million pounds per hour, the maintenance fee for replacing materials, equipment and the process of backwashing and associated power costs for doing so may amount to more than about 1 million dollars annually. In addition to the cost of such service, the filters are not operational during the backwashing and cleaning operation. Consequently, a larger number of filters must be provided in order to provide continuous operation of the power plant. This is an obvious economic disadvantage and, hence, it is desirable to reduce the amount of suspended particles that the filtration system is required to remove in order to prepare effluent brine from the geothermal power plant for injection into the earth.
The present invention is directed to a process for clarifying brine in which the resultant clarified brine has substantially less suspended solids therein than heretofore possible. As can be appreciated from the hereinabove discussion with regard to the cost of filtration of such brine, this results in significant economic advantage in both the reduction of the filtration equipment required and the annual cost of maintaining such filtration equipment in an operational state.
Another advantage of the process of the present invention is the capability of simultaneous production of the seed material for use in the flash crystallizers of a geothermal power plant installation. Hence, the process of the present invention has the advantage of eliminating the process, and apparatus associated therewith, of thickening clarifier underflow in order to prepare it for use in the flash crystallizers as seed material.
Additional advantages and features of the invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawings.